Negative charge image television transmission tube



J. D. M GEE 2 652 535 NEGATIVE CHARGE IMAGE TELEVISION TRANSMISSION TUBE Filed Dec. 18, 1947 invenfw: Jame-s JIM/yer McGee 'orney Patented Sept. 15, 1953 NEGATIVE CHARGE IMAGE TELEVISION TRANSMISSION TUBE James Dwyer McGee, London, England, assignor to Electric & Musical Industries Limited, Hayes, England, a British company Application December 18, 1947, Serial No. 792,576 In Great Britain December 18, 1946 3 Claims.

This invention relates to electron discharge devices, such as television transmission tubes which are used for converting an image of an object into signals for television transmission.

In such devices a target electrode is employed, elemental areas of which are capable of acquiring varying potentials when an optical or electron image is projected on to said electrode, said elemental areas being restored successively to an equilibrium potential by scanning with an electron beam. Usually the arrangement is such that elemental areas of the target electrode are charged positively in proportion to th brightness of the elemental areas of the object for transmission and the target electrode is scanned in one type of television transmission tube with a high velocity beam which restores the elemental areas to an equilibrium potential corresponding substantially to that of the anode of the tube. type, undesirable signals referred to as background and shading are liable to arise since the discharge of the elemental areas of the target electrode is accompanied by secondary electron emission from the target electrode, the undesirable signals being due principally to variations in the secondary emission over the target electrode surface.

The object of the present invention is to provide an improved electron discharge device such as a television transmission tube, with a view to eliminating or substantially reducing the undesirable signals referred to.

According to the present invention there is provided an electron discharg device comprising a target electrode, means whereby a negative charge image can be formed on said target elec trode, means for scanning said target electrode with an electron beam whereby secondary electrons can be released from said target electrode, and means for removing secondary electrons released from said target electrode, the arrangement being suchthat the number of secondary electrons removed from more negatively charged areas of said target electrode is greater than from less negatively charged areas of said target electrode.

According to the present invention there is also provided a circuit arrangement comprising an electron discharg device incorporating a target electrode, means for forming a negative charge image on said target electrode, means for scanning said target electrode with an electron beam at a velocity sufiicient to release secondary elece trons from said target electrode, means ferre- In a television transmission tube of this moving secondary electrons from said target electrode, the arrangement being such that the number of secondary electrons removed from more negatively charged areas of said target electrode is greater than from less negatively charged areas of said target electrode, and means for generating electrical signals representative of said charge image.

If as has been proposed, in television transmission tubes such as referred to in the second paragraph of the specification, the secondary electrons are utilised for generating picture signals, as by directing them to an electron multiplier, the number of secondary electrons which can be so utilised is greater from areas of the target electrode corresponding to dark areas than from bright areas so that the noise tends to be a maximum for dark areas, and in comparison with this the present invention has the advantage that the number of secondary electrons available for generating picture signals is a minimum for dark areas, so that noise should be a minimum for those areas.

In order that the said invention may be clearly understood and readily carried into effect, the

same will now be more fully described with reference to the accompanying drawing wherein a television transmission tube according to one example of the present invention is illustrated.

Referring to the drawing, the tube comprises a highly evacuated glass envelope l with a fiat glass window 2 at one end. On the inner surface of the glass window there is provided a semi-transparent conducting photo-cathode 3 formed for example of an antimony-caesium layer, a lens system indicated at t being arranged to focus anoptical image of the object for transmission onto the photo-cathode 3 and. so liberate photo-electrons therefrom. An anode 5 which may be a metallic coating on the tube wall, is maintained at a potential of about 50 volts positive wtih respect to the photo-cathode 3 which is at earth potential in this example, so that an axial electrostatic field is set up between the photo-cathode 3 and the anode 5 which accelerates th electrons liberated from the photo-cathode. A ring electrode 6 is disposed between the anode 5 and the target electrode l of the tube, and is maintained at or' 'of the tube. region wherethere is no. electrostatic field, the region'being surroundedby a wall anode 9 which is'maintain'ed atthe same potential as the anode A3. and in this regionthe beamis subjected to two' transverse magnetic fields at right angles to positive potential (say 3 voltsy rela tive to that of the photo-cathode.

At the end of the tube I opposite the photocathode 3, but off-set with reference tothe tube axis is arranged an electronzgun consistingof a thermionic cathode l I, a modulator electrode- 42, and an anode I3, and on the application of the potentials indicated to the'respectiye electrodes, an electron beam indicated at l5 is prod uced which emerges from the electron gun with a velocity corresponding -approximately to 150 volts; coining :immediatelycinw the field of in- .fiuence oftheaxial 'magnetic field, produced by :the solenoid .Hl which. as. shown envelope the greaterpropo-rtion .of .thelength of the tube I. -The 'beam l5 .thenpasses. between two so-called lifti plates l4 (onlyone of which isvisible) which 'on' :the application of. a. suitable potential produce an electrostatic field Which.;in conjunction Withthe :axial magneticfielddefiects the beam in knownmanner, so that it :emerges'from between the-plates M at a point-coincident with the axis The beam-then.v passes through a one another and produced by two pairs of saddle coils, one pair :16 of which. is shown. These transversemagnetic fields arearranged to cause theline and frame .scanningdefiections of the beam in known manner, .and on leaving the influence of these fields, sinceit is still subjected 'to theaxial magnetic field. of the coil H], the

beam travels-parallel to the axis of thetube towai'd's the surface of the target electrode 1. However; on approaching the target electrode, the beam enters a deceleratingelectrostatic field between a cylindrical wall anode 9. and the metal meshPBJ Electrons which return fromthetarget follow ap'proximatelythe path ofthe incident electron-"beam l-5 until they reach thelift plates 1 4, where they are deflected in the opposite sense frorn' the incident 'beam and enter an electron multiplierindicatedat |'l and comprising a number of multiplying grids and a collecting electrode which in operation are maintainedat the potentialsindicated. The output of the multiplier forms the signaloutput of the tube and it is fed toth'e input of an amplifier which may be of conventional design and is not shown.

' operation of the tube, the photo-electrons liberated by thephoto cathode 3; are focussed as anelectronimage on the target electrode 1 at such a low velocity that the photo-electrons libcrate veryfew-secondary electrons from the target electrode and hence elemental areas of the glass "surface facing the photocathode 3 are charged negatively in proportion to the brightness of the" elementary areas of the optical image, and hence of theobject'for transmission. These negative chargesjat firstinduce equal negative charges on the opposite surface of the target electrode '5 and then leak through the'glass to equalisethe potential acrossthe thickness of-the-glass. -The time constant of the small condensers constituted by the elemental areas of the target electrode must be smaller than the frame-scan period and no appreciable leakage must occur along the surfaces of the target electrode.

-The scanningselectron beam reaches the target electrode l -through the interstices of the mesh 8, with a velocity corresponding to about .100 volts, sufiicient to liberate more than one secondary electron from the glass surface for ea'chfprimary"electron incident thereon. Providedthe scanned surface of the target electrode lisat a potential-' negative with respect to the m esh 8, these s econdary electrons are accelerated towards themeshfsome being captured by it, but

smost oi;thern pass through and enter the field between, the mesh '8 and the anode 9 and are directedto the electron multiplier ll. This removal of secondary electrons from the target electrode 1 continues vuntil the potential of the scanned surface .lof the target electrode equals or slightly -exceeds the potential of the..mesh 8 -when there is nolfield to accelerate the secondary electrons :through athermesh and acondition of equalibriurn is 1 reached when the. beam of .sec-

- ondary electrons. directed to the .electron multipli'er equalsthe incident scanning beam. Therefore if there is: no light falling on-.the photo- "cathode ii -and hencelno electron imagefocussed on the target electrode i, the latterelectrode will attain an. equilibrium potential approximately equal to the potential. ofithemetal. mesh. However when an .electronsimage. of :the object for transmission is. focussed on the target electrode, elemental areas :thereof are; .as stated, charged negatively in proportion to thebrightness. of the elemental areas ofithe object, andhence when thetarget is scanned secondary electrons are ac- .oelerate'dthrough the mesh 81fromthe elemental secareasof the targetielectrodeithe number of are accelerated through the mesh .8 and. directed tothe electron multiplier .l l trom'elemental areas .of the itarget;...e'lectrode corresponding. to 'the brightiareas of the object for transmissiomand as previously indicated the maximum-input to the. electron; multiplier.ccorresponds .to picture .whiteand the. minimum. input corresponds to picture black. .pIniadditiomthe number of :secondary electrons "accelerated through; the .mesh 8 is not dependent upon variationsin thesesondary. number Iof emissions iromisaid target electrode since thexre'turn beam ofelectrons is controlled by. the. mesh 3 and-the potentials oi? the :target electrode." 7' which.arebrcughtto.the same: relative. potentialratzeach-scan .of 1. the target eleetrode, irrespective-- of variations .sec- .Ondary emission. 1 Theimesl't 3 may therefore be regarded. as atcontrolelectrode. The potential of the .meshi 8. is. arranged to ;be: adjustable over a small range to. compensate for variations at :the

potentialsource.

The. capacityziwhich .is. charged by the. photoelectrons andfidischarged by theelectron .beamzl 5 is that between the target electrode 1 and .the mesh; 8. Sin: thexexarhiale rzillustrated. .I-Ience to obtain. a i sufiicient. ..charge.sstorage' the..-mesh 1 8 must lbe 'disposedivery .;close". to: the target .electrode is. and --.the..: potential swing. of .:.the. target electrode. mustnbel aselarge. as. possible; cf0r, .ex-. ample .about 2..volts. swing. However if: it. is

necessary. to. obtain: greater charge storage, .-.the

sided mosaic screen of the type in which an insulated metal mesh has its interstices filled with conducting plugs. The capacity of the plugs, constituting the elements of the mosaic screen, to the mesh core would then be much greater than between the target electrode and the mesh 8 so that the charges stored would be correspondingly greater, the capacity between the mesh 8 and the double sided mosaic screen being then of secondary importance so that the potential variations could be reduced.

The potential to which an elemental area of the mosaic screen may be charged is determined by the potentials of the photo-cathode 3 and of the mesh 8, and within limits can be increased or decreased by varying the potential of the mesh 8. In the white or high light areas of the image the capacity of the corresponding elemental areas of the target electrode may become fully charged, excess photo-electrons being reflected in that case and returned to the photo-cathode 3. Thus the signal/light characteristic may be approximately linear in the low light areas turning gradually to a saturated portion in the high light areas, which would be suitable for television transmission. If the potential range on the target electrode during transmission is too great, pulling of the incident photo-electrons laterally may occur in the areas corresponding to white. This may however be avoided by using a high capacity double sided mosaic screen such as referred to above; or by using a grid close to the target electrode 1 at the side thereof facing the photo-cathode 3.

In accordance with a modification of the invention, which may result in obtaining a still further improved signal-noise ratio, means may be provided for controlling the secondary electrons which pass the metal mesh 8 in dependence upon the velocity of said secondary electrons, whereby secondary electrons liberated from uncharged areas of the target electrode by the incident beam 15 are prevented from reaching the electron multiplier ll. It will be appreciated that secondary electrons liberated from the charged areas of the target electrode I will have a higher average velocity than those originating from uncharged areas, said average velocity being a maximum for areas corresponding to white. The velocity control may be effected by means of a grid [8 arranged in front of the electron multiplier I? maintained at such potential as to turn back from the multiplier those secondary electrons liberated from uncharged areas of the target electrode l and admit those from charged areas, the grid 18 constituting in effect means for establishing a potential barrier to electrons liberated from uncharged areas. Thus the noise may be reduced in both the low light and high light areas.

Further modifications may of course be made to the embodiment of the present invention which is illustrated; for example the photocathode 3 and target electrode '1 may be constructed in very close proximity. The electrodes 5 and 5 are in that case unnecessary, and the definition of the electron image on the target electrode 1 is determined by the lateral spread of photo-electrons. If the two electrodes 3 and l are sufiiciently close, adequate definition may be obtained in this way and the length of the tube may be appreciably decreased.

What I claim is: r I

1. A circuit arrangement for generating electrical signals corresponding to a light image, in-

eluding a pick-up tube having an evacuated envelope enclosing a target, elemental areas of which are capable of being charged to different potentials, a photoelectric cathode in said envelope for converting a light image into an electron image, circuit connections to cause said electron image to be projected on to said target with a velocity to form a negative charge image on said target, means for scanning said target with an electron beam having a velocity predetermined to release secondary electrons from said target, an electron pervious control electode disposed before the scanned side of said target, means polarizing said control electrode to ac-- celerate said secondary electrons through said control electrode in numbers dependent upon the negative potential of the respective elemental areas of said target, and signal generating means responsive to secondary electrons accelerated through said control electrode for generating electrical signals representative of the potential distribution on the elemental areas of said target with an increase in the number of electrons accelerated through said control electrode corresponding to an increase in the intensity of the light image.

2. Apparatus for generating electrical signals corresponding to a light image, including a pick-up tube having an evacuated envelope enclosing a target, elemental areas of which are capable of being charged to difierent potentials, means including a photoelectric cathode in said envelope for converting a light image to a negative charge image on said target, means for scanning said target with an electron beam having a velocity predetermined to release secondary electrons from said target, an electron pervious control electrode disposed before the scanned side of said target and polarized to accelerate said secondary electrons through said control electrode in numbers dependent upon the negative potential of the respective elemental areas of said target, signal generating means responsive to secondary electrons accelerated through said control electrode for generating electrical signals representative of the potential distribution on the elemental areas of said target with an increase in. the number of electrons accelerated through said control electrode corresponding to an increase in the intensity of the light image, and an electron pervious electrode disposed between said control electrode and said signal generating means polarized to stop secondary electrons released from uncharged areas of said target before said secondary electrons reach said signal generating means.

3. A circuit arrangement for generating electrical signals corresponding to a light image, including a pick-up tube having an evacuated envelope enclosing a target, elemental areas of which are capable of being charged to different potentials, means including a photoelectric cathode in said envelope for converting a light image to an electron image, circuit connections to cause said electron image to be projected on to said target with a velocity to form a negative charge image on said target, means for scanning said target with an electron beam having a velocity predetermined to release secondary electrons from said target, an electron pervious control electrode disposed before the scanned side of said target, means polarizing said control electrode to accelerate said secondary electrons through said control electrode in numbers dependent upon the negative potential of the ree n-saws spective elemental areasef said target .with an Refeppnces; Cited in the file of duh-is patent increase -in -the-number pf ele c trons accelerated EUNITED STATES PATENTS througn sald control electyode eorrespondmg-to j M an increase inthe intensity-of-mhe light image, iiglmber Name 4 .DatelM signal generating means, and means-including .5 551919-87 I u 0 a further control electrode-polarizedto a--pre 3Wmmer 1943 determined potentialfor-idireoting to said-signal generating means all secondary electrons acce1- erated through said control-electrode-with velocities exceeding a-minimum related'to said p1fe 10 i 2i5-f9i5337 aweimer Feb..: 19 1 determined potential.

JAMES DWYER'; MCGEE. 

